1. Field of Invention
The present invention relates generally to semiconductor processing equipment. More particularly, the present invention relates to a pneumatic transfer system which enables air and/or vacuum to be transferred between a coarse stage and a fine stage substantially without introducing disturbance forces related to the pneumatic transfer system.
2. Description of the Related Art
For precision instruments such as photolithography machines which are used in semiconductor processing, factors which affect the performance, e.g., accuracy, of the precision instrument generally must be dealt with and, insofar as possible, eliminated. When the performance of a precision instrument is adversely affected, as for example by pitching moments, products formed using the precision instrument may be improperly formed and, hence, function improperly. For instance, a photolithography machine that is subjected to pitching moments may cause an image projected by the photolithography machine to move, and, as a result, be aligned incorrectly on a projection surface such as a semiconductor wafer surface.
Scanning stages such as wafer scanning stages and reticle scanning stages are often used in semiconductor fabrication processes, and may be included in various photolithography and exposure apparatuses. Wafer scanning stages are generally used to position a semiconductor wafer such that portions of the wafer may be exposed as appropriate for masking or etching. Reticle scanning stages are generally used to accurately position a reticle or reticles for exposure over the semiconductor wafer. Patterns are generally resident on a reticle, which effectively serves as a mask or a negative for a wafer. When a reticle is positioned over a wafer as desired, a beam of light or a relatively broad beam of electrons may be collimated through a reduction lens, and provided to the reticle on which a thin metal pattern is placed. Portions of a light beam, for example, may be absorbed by the reticle while other portions pass through the reticle and are focused onto the wafer.
Many scanning stage devices include a coarse stage and a fine stage which cooperate to position an object such as a reticle or a wafer. Specifically, many high precision machines used in semiconductor fabrication use a coarse stage for relatively large motion and a fine stage for smaller, or more precise, motion. A coarse stage is used to coarsely position a reticle or a wafer near a desired position, while a fine stage is used to finely tune the position of the reticle or the wafer once the reticle or the wafer is positioned near its desired position by the coarse stage.
Some stage devices which include a coarse stage and a fine stage use motors to enable the fine stage to move. The use of mechanical bearings often introduces friction into the stage devices. Hence, in lieu of using mechanical bearings, many stage devices use pneumatic bearings. As shown in FIG. 1a, a coarse stage 104 may be directly coupled to a fine stage 108 through a pneumatic coupling 112. Pneumatic coupling 112 typically includes transfer hoses 114, as shown in FIG. 1b, which enables pneumatics, e.g., pressurized air or a vacuum, to be directly transferred from coarse stage 104 to fine stage 108 for a pneumatic bearing.
Pneumatic coupling 112 provides air and vacuum, as appropriate, to an air bearing (not shown) of fine stage 108 which allows fine stage 108 to move, e.g., scan, on an air bearing surface 120 associated with fine stage 108 and surface 124, although pneumatic coupling 112 may also provide air and vacuum transfer to fine stage 108 for other purposes. As will be understood by those skilled in the art, pressurized air may be provided to an air bearing to cause fine stage 108 to be substantially lifted over surface 124, while a vacuum may be provided to the air bearing to effectively preload fine stage 108.
Pneumatic coupling 112 is generally effective in providing air and/or vacuum to fine stage 108 to allow fine stage 108 to make precise motions. However, since pneumatic coupling 112 is directly coupled between coarse stage 104 and fine stage 108, pneumatic coupling 112 often vibrates when fine stage 108 undergoes a positive acceleration or a negative acceleration, i.e., deceleration. The vibrations in pneumatic coupling 112, e.g., the vibrations of transfer tubes 114, are typically due to the mass of transfer tubes 114 and the stiffness of transfer tubes 114. By way of example, a transfer tube 114 generally has a mass that may create a force during acceleration. Such a force may be transmitted to fine stage 108 as a disturbance or, more specifically, a tube disturbance force. As a result, vibrations which are created in pneumatic coupling 112 may cause transmitted disturbances that compromise the operation of fine stage 108 and the accuracy with which fine stage 108 may be positioned, as such disturbances may cause undesirable vibrations in fine stage 108.
Typically, stiffer transfer tubes 114 create higher tube disturbance forces. It should be understood that even in the event that transfer tubes which directly couple coarse stage 104 to fine stage 108 do not create disturbances, the presence of the transfer tubes applies a constant force to fine stage 108, which may also compromise the operation of fine stage 108.
Substantially eliminating tube disturbance forces which are transmitted to a fine stage or, more generally, substantially eliminating unwanted forces which are experienced by a fine stage, during acceleration of the fine stage, as well as acceleration of a coarse stage, may improve the performance of an overall stage system. Therefore, what is needed is a method and an apparatus for reducing the amount of unwanted forces applied on a fine stage by a pneumatic transfer system. In particular, what is desired is a method and an apparatus for allowing pneumatic transfer to occur between a coarse stage and a fine stage substantially without causing forces such as disturbance forces to be transmitted from a pneumatic transfer system to the fine stage.